Conventional optical repeater circuits require a complex series of both analog and digital signal processing components. For example, an input optical signal is first converted to a photocurrent signal using either p-i-n diodes, m-s-m diodes or avalanche photodetectors. The photocurrent signal is then converted into a small analog voltage by a high speed transimpedance amplifier. The small analog voltage is then amplified by an amplifier and digitized by an analog-to-digital converter to restore the original electronic digital signal. The digital signal is then used to drive a modulator and laser to generate an output optical signal. Therefore, the conventional repeater circuit requires at least six circuit elements to accomplish the task of detecting the intensity of an input light and retransmitting a light of similar intensity as an output light.
In most high speed optical fiber links, data transmission along the fiber is uni-directional and the optical transmitter and receiver, positioned at opposite ends of the fiber, can be independently optimized for high speed operation. However in duplex communication applications, because both a transmitter and receiver are needed at each end of the fiber, the system is substantially more complex. One conventional optical transceiver that perform both the transmission and reception functions uses a transmitting laser as a receiver. Such transceivers cannot operate bi-directionally simultaneously; in other words, such transceivers cannot transmit and receive simultaneously. Other conventional transceiver systems require the use of waveguide splitters to provide separate paths for the transmitted and received optical signals.
It is recently discovered that certain resonant tunneling diodes exhibit opto-electronic characteristics. For example, as discussed in "Optically Switched Resonant Tunneling Diodes," by Moise et al., Applied Physics Letter, Feb. 27, 1995, resonant tunneling diodes can switch between a high conductance and a low conductance electrical state depending on the level of illumination thereon. The switching characteristics are reversible and, in the absence of light, the detector returns to its original high conductance operating state.